Ignition of Nitromethane-based Propellant Mixtures

Hydrazine is a commonly used propellant in spacecraft and satellite propulsion, but it is toxic as well as carcinogenic, and consequently the high costs associated with its handling have led the propulsion community to seek alternatives. Our research group is exploring the use of nitromethane, a low-toxicity substance and a staple laboratory solvent, as the primary ingredient for a green monopropellant. However, nitromethane's ignition is challenging, requiring a certain level of pressure for stable combustion. To improve ignition and combustion behavior, we screened additives including transition metal acetylacetonates and other organometallic compounds. Gas-tight vessels in a differential scanning calorimetry (DSC) device and single-droplet combustion tests were used to measure the influence of additives on the exotherm onset and peak in nitromethane thermal decomposition. The screening showed that some transition-metal acetylacetonates and other organometallic compounds effectively enhance thermal decomposition in sealed-vessel DSC measurements. Ferrocene had the most significant influence: addition of 2 wt. % of this substance to nitromethane caused a decomposition peak shift from 384°C to 280°C. In single-droplet combustion tests in an oxygen-deprived atmosphere at 3 bar, the addition of ferrocene caused a 40% reduction of the droplet ignition delay time, combined with a 20% higher combustion rate compared to neat nitromethane. This makes ferrocene a strong candidate for the use as an ignition and combustion catalyst in nitromethane-based propellants.